Transmission via multiple antennas is an excellent technique for increasing the reliability and the throughput of a communication system. The Multiple Input Multiple Output (MIMO) system is a good example for this technique. In many cases, and in order to increase the throughput and reliability, a single base applies multiple transmitting antennas, each of them is fed by a power amplifier (hereinafter “PA”). In order to obtain the potential gain of said multi-antenna transmission, a spatial precoding and decoding is typically applied at the transmitter (TX) and at the receiver (RX), respectively.
An assumption of a linear channel is commonly adopted in the design of MIMO systems. However, in practice, the MIMO channel may contain nonlinearities, mainly due to the nonlinear power-amplifiers that are used, each of them amplifies the transmitted signal before outputting it to the medium via an antenna. As noted, in order to increase the transmission power, as well as the power efficiency of the PA, it is possible to operate the PA at its non-linear region. However, in this mode of operation the PA distorts the transmitted signal, yielding performance degradation. This performance degradation results in a higher level of error rate, and out-of-band radiation (spectrum widening).
Typically, the existence of saturation levels (i.e., non-linear region) in the PA amplification curve forces the designer of a communication system to choose a proper Back-Off (BO), which influences three major parameters of the communication system: (i) power efficiency, (ii) a metric for spectral purity; and (iii) a level of bit error rate at the receiver. A “back off” is typically defined as the ratio between the maximal emitted power of the PA and the average transmitted power.
While the above problem of non-linear PA exists in most typical communication techniques, it is particularly significant in OFDM transmission and in single carrier transmission with low roll-off factor.
A common approach to reduce the nonlinear distortion is a linearization of the PA curve by compensating for the non-linear region. This approach enables enlargement of the linear dynamic range of the PA and, as a result, reduces the probability of signal distortion. This is a well-established technique, often used in systems with a single TX antenna. This technique may also be applied in a system with multiple TX antennas like in MIMO systems, where the PA in each TX branch can be linearized separately. This approach, however, has a very high implementation complexity, particularly in systems having many TX antennas, e.g., in systems of massive MIMO. Moreover, it seems that the performance of said technique is not optimal, and can be improved in terms of power efficiency, spectral purity, and data reliability.
It is an object of the present invention to provide a transmission structure for increasing the power efficiency of a multi-antenna transmitter and increasing the data throughput of the system, or alternatively reducing the level of error rate. All said advantages are sought to be obtained without harming the spectral purity of the transmitted signals.
It is another object of the present invention to obtain said objects by implementing said transmission structure in a compact digital processing at the transmission side.
It is still another object of the invention to provide said transmission structure without any modification at the receiving side.
It is still another object of the invention to increase by said a transmission structure the efficiency and performance of mobile base stations.
Other objects and advantages of the invention will become apparent as the description proceeds.